Separation membrane

ABSTRACT

A separation membrane ( 10 ) of the present disclosure includes: a separation functional layer ( 30 ) composed of a polyamide containing, as a monomer unit, at least one selected from the group consisting of piperazine and a piperazine derivative; and a coating ( 40 ) covering the separation functional layer ( 30 ) and containing a polymer having a repeating unit represented by the following formula (1). In the formula (1), N +  is a nitrogen atom constituting a quaternary ammonium cation, and R 1  and R 2  are each independently a substituent containing a carbon atom bonded to the nitrogen atom.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to separation membranes.

2. Description of Related Art

Separation membranes are widely used as reverse osmosis membranes (ROmembranes) or nanofiltration membranes (NF membranes) in variousapplications such as production of ultrapure water, desalination ofseawater, and treatment of waste water. Examples of such separationmembranes include composite semipermeable membranes having a poroussupport and a separation functional layer provided on the poroussupport. The separation functional layer is made of an organic compoundsuch as polyamide, polysulfone, and cellulose acetate. In the field ofreverse osmosis membranes, a polyamide membrane obtained bypolymerization of an amine and an acid halide is known to be suitable asthe separation functional layer. The polyamide membrane is typically anaromatic polyamide membrane obtained by interfacial polymerization of anaromatic polyfunctional amine and an aromatic polyfunctional acidhalide. The porous support can be formed of a substrate such as anon-woven fabric and a microporous layer provided on the substrate.

In the field of membrane separation, a separation membrane capable ofselectively separating monovalent ions and divalent ions from each othermay be required. For example, sufficient separation between monovalentions and divalent ions allows easy collection of valuable ions. JP2016-190213 A describes a composite semipermeable membrane that achievesa magnesium sulfate removal ratio of 60% or more.

SUMMARY OF THE INVENTION

The divalent ion selective-separation performance of the compositesemipermeable membrane described in JP 2016-190213 A is not necessarilysatisfactory. Thus, a separation membrane capable of efficient selectiveseparation of divalent ions has been demanded.

The present disclosure provides a separation membrane including:

a separation functional layer composed of a polyamide containing, as amonomer unit, at least one selected from the group consisting ofpiperazine and a piperazine derivative; and

a coating covering the separation functional layer and containing apolymer having a repeating unit represented by the following formula(1).

In the formula (1), N⁺ is a nitrogen atom constituting a quaternaryammonium cation, and R¹ and R² are each independently a substituentcontaining a carbon atom bonded to the nitrogen atom.

The technique of the present disclosure makes it possible to provide aseparation membrane having superior divalent ion selective-separationperformance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a separation membrane according toan embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the present disclosure will be describedwith reference to the drawings. The present disclosure is not limited tothe following embodiment.

As shown in FIG. 1, a separation membrane 10 includes a porous supportmembrane 20, a separation functional layer 30, and a coating 40. Theporous support membrane 20, the separation functional layer 30, and thecoating 40 are stacked in this order. The separation functional layer 30and the coating 40 are supported by the porous support membrane 20. Theseparation functional layer 30 is disposed on the porous supportmembrane 20. The coating 40 is disposed on the separation functionallayer 30. The coating 40 is in contact with the separation functionallayer 30. The separation membrane 10 can be a composite semipermeablemembrane.

The separation functional layer 30 is composed of a polyamidecontaining, as a monomer unit, at least one selected from the groupconsisting of piperazine and a piperazine derivative. Such a polyamideexhibits superior divalent ion selective-separation performance. Thecoating 40 contributes to improvement in the divalent ionselective-separation performance of the separation membrane 10.

In the present specification, the divalent ion selective-separationperformance is a property evaluated by the divalent ion rejection ratioin combination with the difference between the monovalent ion rejectionratio and the divalent ion rejection ratio. When the divalent ionrejection ratio is high and the monovalent ion rejection ratio is low,the divalent ion selective-separation performance can be consideredsuperior. When the difference between the monovalent ion rejection ratioand the divalent ion rejection ratio is small, the divalent ionselective-separation performance cannot be considered superior even ifthe divalent ion rejection ratio is high.

The separation membrane 10 can be produced by the following method.

First, the porous support membrane 20 is prepared as a support. Theporous support membrane 20 is not particularly limited as long as it isa membrane on the surface of which a separation functional layer can beformed. The porous support membrane 20 used may be an ultrafiltrationmembrane having a non-woven fabric on which a microporous layer with anaverage pore diameter of 0.01 to 0.4 μm is formed. Examples of thematerial forming the microporous layer include polyarylethersulfonessuch as polysulfone and polyethersulfone, polyimide, and polyvinylidenefluoride. From the viewpoint of chemical stability, mechanicalstability, and thermal stability, polysulfone or polyarylethersulfonecan be used. A self-supporting porous support membrane having an averagepore diameter as specified above and made of a thermosetting resin suchas epoxy resin can also be used. The thickness of the porous supportmembrane 20 is not particularly limited. The thickness is, for example,in the range of 10 to 200 μm and may be in the range of 20 to 75 μm.

In the present specification, the “average pore diameter” refers to avalue calculated by the following method. First, a surface orcross-section of the membrane or layer is observed with an electronmicroscope (e.g., a scanning electron microscope), and the diameters ofa plurality of observed pores (e.g., 10 randomly selected pores) areactually measured. The average of the actually measured diameters of thepores is defined as the “average pore diameter”. The “diameter of apore” refers to the longest diameter of the pore, and specificallyrefers to the diameter of the smallest of the circles that can enclosethe pore.

Next, a first solution containing a material of the separationfunctional layer 30 is brought into contact with the porous supportmembrane 20. The first solution is typically an aqueous solutioncontaining a polyfunctional amine as the material of the separationfunctional layer 30 (this solution will hereinafter be referred to as“aqueous amine solution”). The contact of the aqueous amine solutionwith the porous support membrane 20 results in the formation of anamine-containing layer on a surface of the porous support membrane 20.The aqueous amine solution may contain, in addition to water, a polarsolvent other than water, such as an alcohol. A polar solvent other thanwater, such as an alcohol, may be used instead of water.

The polyfunctional amine may be at least one selected from the groupconsisting of piperazine and a piperazine derivative. The piperazinederivative is a compound obtained by substitution of at least onehydrogen atom bonded to a carbon atom or nitrogen atom of piperazinewith a substituent. Examples of the substituent include an alkyl group,an amino group, and a hydroxy group. Examples of the piperazinederivative include 2,5-dimethylpiperazine, 2-methylpiperazine,2,6-dimethylpiperazine, 2,3,5-trimethylpiperazine,2,5-diethylpiperazine, 2,3,5-triethylpiperazine, 2-n-propylpiperazine,2,5-di-n-butylpiperazine, and 4-aminomethylpiperazine.

One compound selected from piperazine and the above piperazinederivatives may be used alone as the polyfunctional amine, or acombination of two or more selected from piperazine and the abovepiperazine derivatives may be used as the polyfunctional amine.

In order to facilitate the formation of the amine-containing layer andimprove the performance of the separation functional layer 30, a polymersuch as polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylic acid,or a polyhydric alcohol such as sorbitol and glycerin may be added tothe aqueous amine solution.

The concentration of the amine component in the aqueous amine solutionmay be in the range of 0.1 to 15 wt % and may be in the range of 1 to 10wt %. When the concentration of the amine component is properlyadjusted, the occurrence of defects such as pinholes in the separationfunctional layer 30 can be reduced. Additionally, the separationfunctional layer 30 having high salt rejection performance can beformed. Further, the proper adjustment of the concentration of the aminecomponent leads to proper adjustment of the thickness of the separationfunctional layer 30, thus resulting in the separation membrane 10capable of achieving a sufficient permeation flux.

The method for bringing the aqueous amine solution into contact with theporous support membrane 20 is not particularly limited. A method inwhich the porous support membrane 20 is immersed in the aqueous aminesolution, a method in which the aqueous amine solution is applied to theporous support membrane 20, or a method in which the porous supportmembrane 20 is sprayed with the aqueous amine solution, can be used asappropriate. The step of bringing the aqueous amine solution intocontact with the porous support membrane 20 may be followed by the stepof removing the excess of the aqueous amine solution from the poroussupport membrane 20. For example, the excess of the aqueous aminesolution can be removed from the porous support membrane 20 by extendingthe amine-containing layer with a rubber roller. The removal of theexcess of the aqueous amine solution can result in the formation of theseparation functional layer 30 of appropriate thickness.

Next, a second solution is brought into contact with theamine-containing layer. The second solution is a solution containinganother material of the separation functional layer 30. Specifically,the second solution is a solution containing a polyfunctional acidhalide as the other material of the separation functional layer 30 (thissolution will hereinafter be referred to as “acid halide solution”). Thecontact of the acid halide solution with the amine-containing layerallows a polymerization reaction of the amine and the acid halide toproceed at the interface between the amine-containing layer and a layerof the acid halide solution. Thus, the separation functional layer 30 isformed.

The polyfunctional acid halide is an acid halide having a plurality ofreactive carbonyl groups. Examples of the polyfunctional acid halideinclude an aromatic polyfunctional acid halide, an aliphaticpolyfunctional acid halide, and an alicyclic polyfunctional acid halide.

Examples of the aromatic polyfunctional acid halide include trimesicacid trichloride, terephthalic acid dichloride, isophthalic aciddichloride, biphenyldicarboxylic acid dichloride,naphthalenedicarboxylic acid dichloride, benzenetrisulfonic acidtrichloride, benzenedisulfonic acid dichloride, and chlorosulfonylbenzenedicarboxylic acid dichloride.

Examples of the aliphatic polyfunctional acid halide includepropanedicarboxylic acid dichloride, butanedicarboxylic acid dichloride,pentanedicarboxylic acid dichloride, propanetricarboxylic acidtrichloride, butanetricarboxylic acid trichloride, pentanetricarboxylicacid trichloride, glutaryl halide, and adipoyl halide.

Examples of the alicyclic polyfunctional acid halide includecydopropanetricarboxylic acid trichloride, cyclobutanetetracarboxylicacid tetrachloride, cyclopentanetricarboxylic acid trichloride,cyclopentanetetracarboxylic acid tetrachloride, cyclohexanetricarboxylicacid trichloride, tetrahydrofurantetracarboxylic acid tetrachloride,cyclopentanedicarboxylic acid dichloride, cyclobutanedicarboxylic aciddichloride, cydohexanedicarboxylic acid dichloride, andtetrahydrofurandicarboxylic acid dichloride.

One polyfunctional acid halide selected from these polyfunctional acidhalides may be used alone, or two or more selected from thesepolyfunctional acid halides may be used in combination. An aromaticpolyfunctional acid halide may be used in order to obtain the separationfunctional layer 30 having high salt rejection performance. Apolyfunctional acid halide having three or more valences may be used asat least part of the polyfunctional acid halide component to form across-linked structure.

As the solvent of the acid halide solution there can be used an organicsolvent, in particular a non-polar organic solvent. The organic solventis not particularly limited as long as the organic solvent has lowsolubility in water and can dissolve the polyfunctional acid halidecomponent without deteriorating the porous support membrane 20. Examplesof the organic solvent include saturated hydrocarbons such ascyclohexane, heptane, octane, and nonane and halogen-substitutedhydrocarbons such as 1,1,2-trichlorotrifluoroethane. A saturatedhydrocarbon having a boiling point of 300° C. or lower or 200° C. orlower may also be used.

The concentration of the acid halide component in the acid halidesolution may be in the range of 0.01 to 5 wt % and may be in the rangeof 0.05 to 3 wt %. When the concentration of the acid halide componentis properly adjusted, the amounts of the amine and halide componentsremaining unreacted can be reduced. Additionally, the occurrence ofdefects such as pinholes in the separation functional layer 30 can bereduced, and thus the separation membrane 10 having high salt rejectionperformance can be provided. Further, the proper adjustment of theconcentration of the acid halide component leads to proper adjustment ofthe thickness of the separation functional layer 30, thus providing theseparation membrane 10 capable of achieving a sufficient permeationflux.

The method for bringing the acid halide solution into contact with theamine-containing layer is not particularly limited. The amine-containinglayer may be immersed in the acid halide solution together with theporous support membrane 20, or the acid halide solution may be appliedto a surface of the amine-containing layer. The time of contact betweenthe amine-containing layer and the acid halide solution is, for example,10 seconds to 5 minutes or 30 seconds to 1 minute. The contact betweenthe amine-containing layer and the acid halide solution may be followedby the step of removing the excess of the acid halide solution from theamine-containing layer.

Next, the separation functional layer 30 is heated and dried togetherwith the porous support membrane 20. The heat treatment of theseparation functional layer 30 can improve the properties such as themechanical strength and the heat resistance of the separation functionallayer 30. The heating temperature is, for example, 70 to 200° C. or 80to 130° C. The heating time is, for example, 30 seconds to 10 minutes or40 seconds to 7 minutes. A drying step may be carried out at a roomtemperature, and subsequently another drying step may be carried outusing a dryer at an ambient temperature higher than the roomtemperature.

The conditions under which interfacial polymerization is carried out aredescribed, for example, in JP 58-24303 A and JP 1-180208 A. Such knowntechniques can be employed in the method of the present embodiment.

Various additives can be added to the aqueous amine solution and/or theacid halide solution in order to facilitate the formation of theseparation functional layer 30 or improve the performance of theseparation membrane 10 to be obtained. Examples of the additivesinclude: a surfactant such as sodium dodecylbenzenesulfonate, sodiumdodecyl sulfate, and sodium lauryl sulfate; a basic compound, such assodium hydroxide, trisodium phosphate, and triethylamine, which has aneffect on removal of halogenated hydrogen produced as a result ofpolymerization; an acylation catalyst; and a compound as described in JP8-224452 A which has a solubility parameter of 8 to 14 (cal/cm³)^(1/2).

By carrying out the above steps, a membrane having the porous supportmembrane 20 and the separation functional layer 30 is obtained. Thethickness of the separation functional layer 30 is not particularlylimited. The thickness is, for example, 0.05 to 2 μm and may be 0.1 to 1μm.

The present specification describes a method in which the separationfunctional layer 30 is formed directly on a surface of the poroussupport membrane 20 by interfacial polymerization. It should be notedthat the separation functional layer 30 may be formed on a support otherthan the porous support membrane 20, and the separation functional layer30 obtained may be placed on and joined to the porous support membrane20. In other words, the separation functional layer 30 may betransferred onto the porous support membrane 20 from the other support.

Next, a solution containing a material of the coating 40 is brought intocontact with the separation functional layer 30. The material of thecoating 40 can be a polymer having a repeating unit represented by thefollowing formula (1).

In the formula (1), N⁺ is a nitrogen atom constituting a quaternaryammonium cation. R¹ and R² are each independently a substituentcontaining a carbon atom bonded to the nitrogen atom.

When the coating 40 contains the polymer having the repeating unitrepresented by the formula (1), the divalent ion selective-separationperformance of the separation membrane 10 is improved. The quaternaryammonium cation is always positively charged irrespective of the pH ofraw water. This prevents adhesion of cationic substances to the surfaceof the separation membrane 10.

In the formula (1), the counterion for N⁺ is not particularly limited.The counterion for N⁺ is a monovalent anion. Examples of the monovalentanion include halogen ions such as F⁻, Cl⁻, Br⁻, and I⁻.

In the formula (1), R¹ and R² may each be an alkyl group. Examples ofthe alkyl group include a methyl group, an ethyl group, and a propylgroup. In particular, R¹ and R² may each be a methyl group. When R¹ andR² are each an alkyl group such as a methyl group, the coating 40 cansufficiently improve the divalent ion selective-separation performanceof the separation membrane 10. When R¹ and R² are each an alkyl groupsuch as a methyl group, the coating 40 causes less influence on the saltrejection ratio of the separation membrane 10.

In the formula (1), R¹ may be a methyl group, while R² may be a3-chloro-2-hydroxypropyl group. In this case, the repeating unit of thepolymer is represented by the following formula (2).

In the formula (1), R¹ may be a methyl group, while R² may be a2,3-epoxypropyl group. In this case, the repeating unit of the polymeris represented by the following formula (3).

When an alkali is allowed to act on the repeating unit represented bythe formula (2), the 3-chloro-2-hydroxypropyl group undergoes acyclization reaction. This reaction converts the repeating unitrepresented by the formula (2) to the repeating unit represented by theformula (3).

The polymer contained in the coating 40 can be a copolymer of a firstmonomer and a second monomer. The first monomer can be a monomercontaining a quaternary ammonium cation and serving to form therepeating unit represented by the formula (1). The first monomer can be3-chloro-2-hydroxypropylmethyl diallyl ammonium chloride. When thesecond monomer is diallylmethylamine hydrochloride, the copolymer isrepresented by the following formula (4).

In the formula (4), m and n are each independently an integer of 1 ormore. Part or all of the 3-chloro-2-hydroxypropyl groups contained inthe polymer of the formula (4) may be a 2,3-epoxypropyl group as shownin the formula (3).

In the copolymer represented by the formula (4), the3-chloro-2-hydroxypropyl group and/or the 2,3-epoxypropyl groupcontained in the first monomer can be a reactive substituent capable ofbeing chemically bonded to the separation functional layer 30.

The polymer of the formula (4) can be obtained also by modification of ahomopolymer. Specifically, a homopolymer of methyl diallylaminehydrochloride can be modified with epichlorohydrin to obtain the polymerof the formula (4).

The reactive substituent enhances the bond strength between theseparation functional layer 30 and the coating 40. Specifically, atleast part of the reactive substituents form a covalent bond with aterminal amino group, a remaining amino group, or a remaining carbonylgroup of the separation functional layer 30. Thus, the coating 40 issecured to the separation functional layer 30, so that the separationmembrane 10 resistant to deterioration of divalent ionselective-separation performance even in long-term use can be provided.The terminal amino group and the remaining amino group of the separationfunctional layer 30 are derived from the polyfunctional amine. Theremaining carbonyl group of the separation functional layer 30 isderived from the polyfunctional acid halide. The reactive substituentmay be used in intramolecular cross-linking and/or intermolecularcross-linking of the polymer. Such cross-linking can improve theproperties such as the mechanical strength and the heat resistance ofthe coating 40.

The reactive substituent may be contained in the second monomer. Whenthe first monomer has a quaternary ammonium cation structure, fewerrestrictions are imposed on the second monomer. That is, there is a lotof flexibility in choosing the second monomer.

The reactive substituent is not limited to a 3-chloro-2-hydroxypropylgroup. Examples of the reactive substituent include an epoxy group, ahydroxy group, an amino group, and an amide group. One substituentselected from these reactive substituents may be contained alone in thepolymer, or two or more selected from these reactive substituents may becontained in the polymer.

When the reactive substituent is contained in the second monomer, thesecond monomer is, for example, allylamine. The polymer is representedby the following formula (5).

In the formula (5), m and n are each independently an integer of 1 ormore.

When the reactive substituent is contained in the second monomer, thesecond monomer is, for example, acrylamide. The polymer is representedby the following formula (6).

In the formula (6), m and n are each independently an integer of 1 ormore.

When the reactive substituent is contained in the second monomer, thesecond monomer is, for example, 3-chloro-2-hydroxypropyl diallyaminehydrochloride. The polymer is represented by the following formula (7).

In the formula (7), m and n are each independently an integer of 1 ormore.

One monomer selected from 3-chloro-2-hydroxypropyl diallylaminehydrochloride, allylamine, and acrylamide may be used alone as thesecond monomer, or two or more selected from these monomers may be usedas the second monomer.

The copolymer may be a random copolymer or a block copolymer.

The ratio between the first monomer and the second monomer is notparticularly limited. For example, the ratio (first monomer:secondmonomer) is 5:95 to 95:5 and may be 30:70 to 70:30. When the ratio iswithin this range, the separation membrane 10 having superior divalention selective-separation performance and usable over a long period oftime can be provided. The weight-average molecular weight of the polymeror copolymer is not particularly limited and is, for example, 10,000 to100,000.

The coating 40 can be formed by bringing an aqueous solution containingthe polymer into the separation functional layer 30 to form apolymer-containing layer and then drying the polymer-containing layer.The method for bringing the aqueous solution into contact with theseparation functional layer 30 is not particularly limited. Theseparation functional layer 30 may be immersed in the aqueous solutiontogether with the porous support membrane 20, or the aqueous solutionmay be applied to a surface of the separation functional layer 30. Thetime of contact between the separation functional layer 30 and theaqueous solution is, for example, 10 seconds to 5 minutes. The contactbetween the separation functional layer 30 and the aqueous solution maybe followed by the step of removing the excess of the aqueous solutionfrom the separation functional layer 30. The aqueous solution maycontain, in addition to water, a polar solvent other than water, such asan alcohol. A polar solvent other than water, such as an alcohol, may beused instead of water.

Next, the polymer-containing layer is heated and dried. The heattreatment of the polymer-containing layer can improve the propertiessuch as the mechanical strength and the heat resistance of the coating40. The heating temperature is, for example, 80 to 150° C. The heatingtime is, for example, 30 to 300 seconds. A drying step may be carriedout at a room temperature, and subsequently another drying step may becarried out using a dryer at an ambient temperature higher than the roomtemperature.

By carrying out the above steps, the separation membrane 10 having theporous support membrane 20, the separation functional layer 30, and thecoating 40 is obtained. The thickness of the coating 40 is notparticularly limited and is, for example, 10 to 900 nm. The presence ofthe coating 40 can be confirmed by means of a transmission electronmicroscope. The composition analysis of the polymer contained in thecoating 40 can be carried out by Fourier-transform infrared spectroscopy(FT-IR), X-ray photoelectron spectroscopy (XPS), or time-of-flightsecondary ion mass spectrometry (TOF-SIMS).

EXAMPLES Example 1

An aqueous amine solution containing 7 wt % of piperazine, 0.15 wt % ofsodium dodecyl sulfate, 1.48 wt % of sodium hydroxide, and 6 wt % ofcamphorsulfonic acid was applied to a porous polysulfone support. Afterthat, the excess of the aqueous amine solution was removed from thesupport to form an amine-containing layer on the support. Next, thesurface of the amine-containing layer was immersed for 10 seconds in anacid halide solution obtained by solving 0.42 wt % of trimesic acidtrichloride in an isoparaffinic solvent (IP Solvent 1016, manufacturedby Idemitsu Kosan Co., Ltd.). After that, the excess of the acid halidesolution was removed from the amine-containing layer, and theamine-containing layer was air-dried for 60 seconds and then placed in ahot air dryer at 120° C. for 3 minutes to form a separation functionallayer on the porous polysulfone support. Next, the surface of theseparation functional layer was immersed for 10 seconds in an aqueoussolution containing 0.1 wt % of a polymer (UNISENCE KCA 101L,manufactured by SENKA Corporation). After that, the separationfunctional layer was air-dried for 30 seconds and then placed in a hotair dryer at 120° C. for 2 minutes to form a coating on the separationfunctional layer. In this manner, a separation membrane of Example 1 wasobtained. UNISENCE KCA 101L is a polymer represented by the formula (4).

Example 2

A separation membrane was obtained in the same manner as in Example 1,except that the concentration of the polymer in the aqueous solution waschanged to 0.03 wt %.

Example 3

A separation membrane was obtained in the same manner as in Example 1,except that the concentration of the polymer in the aqueous solution waschanged to 0.01 wt %.

Example 4

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to PAS-880 manufactured by NittoboMedical Co., Ltd. and that the concentration of the polymer in theaqueous solution was changed to 0.05 wt %. PAS-880 is a copolymerrepresented by the formula (7). R¹ and R² are each a methyl group.

Example 5

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to PAS-880 manufactured by NittoboMedical Co., Ltd.

Example 6

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to PAS-J-81 manufactured by NittoboMedical Co., Ltd. PAS-J-81 is a copolymer represented by the formula(6). R¹ and R² are each a methyl group.

Example 7

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to PAA-1123 manufactured by NittoboMedical Co., Ltd. PAA-1123 is a copolymer represented by the formula(5). R¹ and R² are each a methyl group.

Example 8

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to PAS-H-5L manufactured by NittoboMedical Co., Ltd. and that the concentration of the polymer in theaqueous solution was changed to 0.05 wt %. PAS-H-5L is a polymerrepresented by the formula (1). R¹ and R² are each a methyl group. Thatis, PAS-H-5L is a homopolymer of diallyldimethylammonium chloride.

Example 9

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to a mixture of the polymers used inExamples 1 and 4 and that the respective concentrations of the polymersin the aqueous solution were 0.03 wt % and 0.02 wt %.

The polymers used in Examples 4 to 9 belong to diallyldimethylammoniumchloride polymers.

Comparative Example 1

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to polyvinyl alcohol.

Comparative Example 2

A separation membrane was obtained in the same manner as in Example 1,except that the polymer was changed to polyquaternium-10 (CATINALHC-100, manufactured by TOHO Chemical Industry Co., Ltd.). CATINALHC-100 is cellulose containing quaternary ammonium cation.

Comparative Example 3

A separation membrane was obtained in the same manner as in Example 1,except that no coating was formed on the surface of the separationfunctional layer.

[Performance Evaluation: Rejection Ratio]

The MgSO₄ rejection ratio and the NaCl rejection ratio were measured forthe obtained separation membranes. In the measurement of the MgSO₄rejection ratio, an aqueous MgSO₄ solution having a temperature of 25°C., a pH of 6.5 to 7, and a concentration of 2000 mg/liter was used. Inthe measurement of the NaCl rejection ratio, an aqueous NaCl solutionhaving a temperature of 25° C., a pH of 6.5 to 7, and a concentration of2000 mg/liter was used.

The aqueous MgSO₄ solution or aqueous NaCl solution was passed througheach of the separation membranes of Examples and Comparative Examples atan operating pressure of 1.5 MPa for 30 minutes. An electricalconductivity meter (CM-117, manufactured by Kyoto ElectronicsManufacturing Co., Ltd.) was used to measure the electrical conductivityof the permeate liquid and the feed liquid. From the measurement resultand a calibration curve (concentration versus electrical conductivity),the MgSO₄ rejection ratio and the NaCl rejection ratio were calculatedaccording to the following equations. The results are shown in Table 1.

MgSO₄ rejection ratio (%)=(1−(MgSO₄ concentration of permeateliquid/MgSO₄ concentration of feed liquid))×100

NaCl rejection ratio (%)=(1−(NaCl concentration of permeate liquid/NaClconcentration of feed liquid))×100

TABLE 1 MgSO₄ NaCl Concentration rejection rejection Type of of polymerratio ratio polymer (%) (%) (%) Example 1 UNISENCE KCA 0.10 99.8 59.7101L Example 2 UNISENCE KCA 0.03 99.8 60.5 101L Example 3 UNISENCE KCA0.01 99.8 60.5 101L Example 4 PAS-880 0.05 99.8 55.0 Example 5 PAS-8800.10 99.8 52.0 Example 6 PAS-J-81 0.10 99.8 57.0 Example 7 PAA-1123 0.1099.8 57.0 Example 8 PAS-H-5L 0.05 99.6 55.0 Example 9 UNISENCE KCA 0.0399.8 56.0 101L 0.02 PAS-880 Comparative Polyvinyl alcohol 0.10 99.8 70.5Example 1 Comparative Polyquaternium- 0.10 99.7 71.0 Example 2 10Comparative Not applicable — 99.7 71.5 Example 3

As shown in Table 1, the separation membranes of Examples 1 to 9 andComparative Examples 1 to 3 exhibited the same level of MgSO₄ rejectionratio.

The NaCl rejection ratio of the separation membranes of Examples 1 to 9was lower than the NaCl rejection ratio of the separation membranes ofComparative Examples 1 to 3. The NaCl rejection ratio of the separationmembranes of Example 1 to 9 was 60.5% at a maximum. The NaCl rejectionratio of the separation membranes of Comparative Examples 1 to 3 was70.5% or more. For the separation membranes of Examples 1 to 9, thedifference between the MgSO₄ rejection ratio and the NaCl rejectionratio was large. This means that the separation membranes of Examples 1to 9 exhibited superior divalent ion selective-separation performance.

The separation membranes of Comparative Examples 1 and 2, despite havinga surface coating, exhibited a NaCl rejection ratio similar to that ofthe separation membrane of Comparative Example 3 which had no surfacecoating. That is, the surface coatings of the separation membranes ofComparative Examples 1 and 2 made no contribution to improvement indivalent ion selective-separation performance.

The separation membrane of the present disclosure can be used as a RO(reverse osmosis) membrane, a NF (nanofiltration) membrane, an UF(ultrafiltration) membrane, a MF (microfiltration) membrane, or a FO(forward osmosis) membrane.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this specification are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A separation membrane comprising: a separation functional layer composed of a polyamide containing, as a monomer unit, at least one selected from the group consisting of piperazine and a piperazine derivative; and a coating covering the separation functional layer and containing a polymer having a repeating unit represented by the following formula (1):

wherein N⁺ is a nitrogen atom constituting a quaternary ammonium cation, and R¹ and R² are each independently a substituent containing a carbon atom bonded to the nitrogen atom.
 2. The separation membrane according to claim 1, wherein in the formula (1), R¹ and R² are each an alkyl group.
 3. The separation membrane according to claim 1, wherein in the formula (1), R¹ and R² are each a methyl group.
 4. The separation membrane according to claim 1, wherein in the formula (1), R¹ is a methyl group, and R² is a 3-chloro-2-hydroxypropyl group.
 5. The separation membrane according to claim 1, wherein in the formula (1), R¹ is a methyl group, and R² is a 2,3-epoxypropyl group.
 6. The separation membrane according to claim 1, wherein the polymer is a copolymer of a first monomer serving to form the repeating unit and a second monomer, and the second monomer has a reactive substituent capable of being chemically bonded to the separation functional layer.
 7. The separation membrane according to claim 6, wherein the reactive substituent is an epoxy group, a hydroxy group, an amino group, or an amide group.
 8. The separation membrane according to claim 6, wherein the second monomer is 3-chloro-2-hydroxypropyl diallylamine hydrochloride, allylamine, or acrylamide.
 9. The separation membrane according to claim 1, further comprising a support supporting the separation functional layer. 